Method and device for marine seismic acquisition

ABSTRACT

Method and system for improving offset/azimuth distribution. The system includes plural streamers towed by a streamer vessel; a central source towed by the streamer vessel; first and second front sources located in front of the plural streamers along a traveling direction of the streamer vessel; and first and second large offset front sources located in front of the first and second front sources along the traveling direction. The offset distance between the first and second large offset front sources, along a cross-line direction, is larger than an offset distance between the first and second front sources.

RELATED APPLICATION

The present application is a Continuation of U.S. patent applicationSer. No. 14/185,199, filed Feb. 20, 2014, entitled “Method and Devicefor Marine Seismic Acquisition”, which is a Continuation of U.S. patentapplication Ser. No. 13/469,376 filed on May 11, 2012, entitled “Methodand Device for Marine Seismic Acquisition”, which is related to, andclaims priority from, U.S. Provisional Patent Application No.61/557,541, filed on Nov. 9, 2011, entitled “Method and Device forMarine Seismic Acquisition”, the entire contents of which areincorporated herein by reference.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor improved azimuth distribution in seismic data acquisition.

2. Discussion of the Background

Marine seismic data acquisition and processing generate a profile(image) of a geophysical structure (subsurface) under the seafloor. Thisprofile does not necessarily provide an accurate location for oil andgas reservoirs, but it may suggest, to those trained in the field, thepresence or absence of oil and/or gas reservoirs. Thus, providing ahigh-resolution image of the subsurface is an ongoing process.

For a seismic gathering process, as shown in FIG. 1, a data acquisitionsystem 10 includes a vessel 12 towing plural streamers 14 that mayextend over kilometers behind the vessel. One or more source arrays 16may be also towed by the vessel 12 or another vessel for generatingseismic waves. Conventionally, the source arrays 16 are placed in frontof the streamers 14, considering a traveling direction of the vessel 12.The seismic waves generated by the source arrays propagate downward andpenetrate the seafloor, eventually being reflected by a reflectingstructure (not shown) back to the surface. The reflected seismic wavespropagate upwardly and are detected by detectors provided on thestreamers 14. However, such a method results in data having poor azimuthdistribution.

An improvement to this conventional data acquisition method is the useof wide-azimuth (WAZ) acquisition. In a typical WAZ survey, two streamervessels and multiple sources are used to cover a large sea area, and allsources and streamers are controlled at a uniform depth throughout thesurvey. WAZ acquisition provides better illumination of the substructureand, thus, a better final image. However, the presence of ghosts (e.g.,reflections of waves from the surface of the water back to the receiversof the streamers) in the acquired data still affects the final image dueto the presence of notches.

A newer approach, rich-azimuth (RAZ) acquisition, shows promising signsfor improving the final image. RAZ acquisition is the combination ofmulti-azimuth acquisition and wide-azimuth geometry. RAZ acquisition maybe implemented by shooting a same cell in three directions, e.g., 30°,90°, and 150°, each direction being shot in one or two passes. A rosediagram for such a rich-azimuth survey shows the benefits ofrich-azimuth towed-streamer acquisition, i.e., azimuth coverage from 0°to 360° and uniform offset distribution from 400 m to 8000 m for a 8000m long streamer.

However, the existing RAZ acquisition can further be improved toincrease the illumination and accuracy of the final image by finding anappropriate number and distribution of source arrays to achieve ultralong offset together with broadband techniques. Accordingly, it would bedesirable to provide systems and methods that avoid the afore-describedproblems and drawbacks, and improve the accuracy of the final image.

SUMMARY

According to an exemplary embodiment, there is a survey acquisitionsystem that includes plural streamers towed by a streamer vessel; acentral source towed by the streamer vessel; first and second frontsources located in front of the plural streamers along a travelingdirection of the streamer vessel; and first and second large offsetfront sources located in front of the first and second front sourcesalong the traveling direction. An offset distance between the first andsecond large offset front sources, along a cross-line direction (Y), islarger than an offset distance between the first and second frontsources.

According to another exemplary embodiment, there is a survey acquisitionsystem that includes plural streamers towed by a streamer vessel; acentral source towed by the streamer vessel; first and second frontsources located in front of the plural streamers along a travelingdirection of the streamer vessel; and first and second large offsetfront sources located in front of the first and second front sourcesalong the traveling direction. An offset distance between the secondlarge offset front source and the traveling distance, along a cross-linedirection (Y), is larger than an offset distance between the first frontsource and the traveling distance. The offset distance between thesecond large offset front source and the traveling distance, along across-line direction (Y), is larger than an offset distance between thesecond front source and the traveling distance.

According to still another exemplary embodiment, there is a method forseismic data acquisition that includes towing plural streamers with astreamer vessel; towing a central source with the streamer vessel;towing first and second front sources, located in front of the pluralstreamers along a traveling direction of the streamer vessel, withcorresponding front towing vessels; and towing first and second largeoffset front sources, located in front of the first and second frontsources along the traveling direction, with corresponding large offsetfront towing vessels. The first and second front sources, the centralsource and the first and second large offset front sources are actuatingsimultaneously or sequentially during the survey.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a conventional seismic data acquisitionsystem;

FIG. 2 is a schematic diagram of a novel seismic data acquisition systemaccording to an exemplary embodiment;

FIG. 3 is a schematic diagram of another novel seismic data acquisitionsystem according to an exemplary embodiment;

FIG. 4 is a schematic diagram of a curved streamer and a large offsetsource vessel according to an exemplary embodiment;

FIG. 5 is a schematic diagram of a seismic data acquisition systemhaving plural source vessels in front of streamers according to anexemplary embodiment;

FIG. 6 is a schematic diagram of a seismic data acquisition systemhaving plural source vessels in front of streamers and asymmetriclocated according to an exemplary embodiment;

FIG. 7 is a flowchart of a method for towing multiple sources withmultiple vessels according to an exemplary embodiment; and

FIG. 8 is a schematic diagram of a computerized system that implementsvarious methods according to an exemplary embodiment.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of RAZ acquisition using a vessel streamer and five sources.However, the embodiments to be discussed next are not limited to theseconfigurations, but may be extended to other arrangements as discussedlater.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

According to an exemplary embodiment, a system configuration forenriching RAZ acquisition includes a streamer vessel configured to towplural streamers and a source array and plural source vessels configuredto tow one or more source arrays. Two source vessels may be configuredto sail parallel to the streamer vessel, substantially at a sameposition along a traveling direction of the streamer vessel. Two othersource vessels are configured to sail either in front of the streamervessel or behind the streamers of the streamer vessel. In oneapplication, two source vessels (front source vessels) are providedahead of the streamers, e.g., next to the streamer vessel while theother two source vessels (large offset source vessels) are providedfurther ahead of the streamer vessel along the traveling direction.

According to another exemplary embodiment, the large offset sourcevessels may be provided with a larger cross-line separation than thefront source vessels. In still another exemplary embodiment, the largeoffset source vessels may be provided symmetrically or asymmetricallyrelative to the traveling direction. In yet another exemplary embodimentthe streamers may be provided to have a dovetail-like (fan) arrangement.According to still another exemplary embodiment, the streamers may havea variable depth along as described, for example, in patent applicationSer. No. 13/272,428, entitled “Method and Device to Acquire MarineSeismic Data,” and authored by R. Soubaras, the entire content of whichis incorporated herein by reference. Still, in another application, notwo source arrays are at the same inline position along the travelingdirection. These embodiments are now discussed in more detail below.

According to an exemplary embodiment illustrated in FIG. 2, there is aseismic acquisition system 100 that includes a streamer vessel 102 andfour source vessels 104, 106, 108 and 110. The streamer vessel 102 towsplural streamers 112 and, optionally, a source array 114. The sourcevessels tow corresponding source arrays 104 a, 106 a, 108 a, and 110 a.The source arrays may include one or more individual sources. Anindividual source may be, for example, an air gun. The streamers 112 aresubstantially parallel in this embodiment. However, as shown in FIG. 3,the streamers 112 may be distributed in a dovetail-like shape. In oneapplication, the streamers 112 are fanned in a horizontal plane(substantially parallel to the water surface) so that they make an anglewith each other. To achieve this arrangement, birds 113 may be locatedon each streamer 112 as shown in FIG. 3, for maintaining the streamersat the desired positions. The birds are devices capable of maintaining avertical and/or horizontal position in water.

Returning to FIG. 2, it is noted that the X axis corresponds to thetraveling direction of the vessels, also known in the art as the inline,and the Y axis, which is perpendicular to X axis, is known in the art asthe cross-line. With this convention, a cross-line distance D1 betweensources 104 a and 106 a (front sources) may be approximately 1200 mwhile a cross-line distance D2 between sources 108 a and 110 a (tailsources) may be approximately 2400 m. A central source 114 may be placedat half distance between the front sources. These numbers are exemplaryand not intended to limit the exemplary embodiments. However, acharacteristic of this exemplary embodiment is that the separationdistance between the tail sources is larger than the separation distancebetween the front sources. In one application, the separation distancebetween the tail sources is substantially double the separation distanceof the front sources.

Another characteristic of this exemplary embodiment is the inlinedistance between the sources. Considering the front sources 104 a and106 a, it is noted that there is an inline displacement D_(HI), betweenthem. The central source 114 may also be displaced inline (e.g., D_(CI))relative to one of the front sources. A similar inline displacementD_(TI) may be implemented for the tail sources 108 a and 110 a. Thevalues for these inline displacements vary from survey to survey,depending on various factors such as, for example, length of streamers,number of streamers, depth of sea bottom, etc.

The streamers 112 may be towed to be substantially parallel or slantedto the water surface. As shown in FIG. 2, the streamers may have alength D3+D4 (to be explained later) and an offset between an end of thestreamer and the tail source 108 a is D5. In one application, thestreamers may have a curved profile, as shown in FIG. 4 (e.g., Broadseisconfiguration originated by CGGVeritas, France). Supposing that thestreamer 112 shown in FIG. 4 has a length of, for example, 10 km, thecurved portion 112 a may have a length D3=2 km and the flat portion 112b may have a length D4=8 km. For these specific values, an offset of thetail source 108 a relative to an end of the streamer (along the X axis)is about D5=8 km, i.e., substantially equal to the flat portion of thestreamer. This is considered a large offset in the industry. Asdisclosed in various patent applications of CGGVeritas, lengths of thecurved portion and the flat portion differ from survey to survey and,thus, the numbers disclosed in this embodiment are exemplary. Moreover,such embodiments do not require a linkage between the source positionand the flat portion of the streamer. For example, the configuration ofFIG. 2 can also be applied to embodiments where one or more streamers donot have flat portions and the same source positions as shown in FIG. 2may still be implemented in the overall configuration.

For a 10 km long streamer with a flat portion of 8 km, it is expectedthat the tail source should be 8 km away from the end of the streamer toavoid having shallow tow data in the long offset gathers when using theBroadseis method. Further, this offset also allows for feathervariations to be minimized.

The five sources may be fired using various schemes. One scheme is toshoot the sources sequentially, for example, at 37.5 m intervals (i.e.,shoot a first front source, wait for the first front source to travel37.5 m along the X axis, and then shoot the central source, and so on).The value of 37.5 m is exemplary and is based on the traveling speed ofthe streamer vessel. In this way, the sources are fired when they havethe same inline position during a firing sequence. A firing sequenceincludes the sequential firing of each source once. Another scheme is toshoot the sources almost instantaneously, with random time delays. It isnoted that for a 20 km offset, the tail sources need to be shot untilthe end of the full-fold boundary. Still another scheme is to shoot thesources at the same times.

According to another exemplary embodiment illustrated in FIG. 5, aseismic system 200 includes a streamer vessel 202 and four sourcevessels 204, 206, 208, and 210. In one application, less or more sourcevessels may be used. The streamer vessel 202 is configured to tow pluralstreamers 212, which may be substantially parallel to each other orhaving a dovetail configuration, as shown in FIG. 3. The streamer vessel202 may also be configured to tow a source 214 along a travelingdirection 216.

Two (204 and 206) of the four source vessels are configured to movesubstantially in parallel with the streamer vessel 202. These two sourcevessels (front source vessels) are located in front of the streamers212. Each of the front source vessel 204 and 206 is configured to tow atleast a front source (204 a or 206 a) that is also positioned in frontof the streamers 212. Sources 214, 204 a and 206 a may be located tohave the same in-line coordinate (i.e., same value on X axis). Inanother application, which is illustrated in FIG. 5, the front source204 a is ahead of the source 214 by a distance D1 and the source 214 isahead of the front source 206 a by a distance D2. In one application, D1may be equal with D2.

Source vessels 208 and 210 (called herein “large offset front sources”as an offset distance between them is large, e.g., 2.4 km) may belocated in front of the vessels 202, 204 and 206 and each of thesesource vessels may tow at least one source 208 a or 210 a. In oneapplication, a distance D3 between (i) the large offset front sources208 a and 210 a and (ii) the front sources 204 a and 206 a is in theorder of kms, for example, 2 to 10 km. Of course, smaller or largeroffsets are also possible depending on the characteristics of thesurvey. In one application, the large offset front sources 208 a and 210a are separated by an inline distance D4, which may have a value of zeroto hundreds of meters.

The configuration shown in FIG. 5 has the four sources 204 a, 206 a, 208a and 210 a disposed symmetrically relative to the traveling direction216. Further, a half cross-line separation between the front sources 204a and 206 a and the traveling distance 216 is D5 and a half cross-lineseparation between the large offset front sources 208 s and 210 a andthe traveling distance 216 is D6, which is larger than D5. In oneapplication, D5 is about 1200 m and D6 is about 2400 m. For thisexemplary embodiment, the streamers 212 may be 10 km long and inlineseparated by about 100 m. In one application, the plurality of streamers212 includes 10 streamers.

According to still another exemplary embodiment illustrated in FIG. 6,there is a seismic system 300 that includes a streamer vessel 302 andfour source vessels 304, 306, 308 and 310. More or less source vesselsmay be used. Each of the vessels may tow a corresponding seismic source302 a, 304 a, 306 a, 308 a and 310 a and the streamer vessel 302 alsotows a plurality of streamers 312. The streamers configuration may besimilar to the embodiment disclosed in FIG. 5. Further, the streamersshown in FIGS. 5 and 6 may have the configuration shown in FIGS. 3and/or 4. In this embodiment, the front sources 304 a and 306 a aredisposed on a same side of the streamer vessel 302 relative to thetraveling direction 314, the front source vessel 304 being separated bya cross-line distance A1 from the streamer vessel 302 and the frontsource vessel 306 being separated by a cross-line distance A1+A2 fromthe streamer vessel 302. In one application, each of the cross-linedistances A1 and A2 may be around 1200 m. The large offset sourcevessels 308 and 310 may also be provided on the same side of thestreamer vessel 302 relative to the traveling direction 314, similar tothe front sources 304 and 306. In this exemplary embodiment, across-line distance between the traveling direction 314 and the largeoffset vessel 308 is A3 and a cross-line distance between the travelingdirection 314 and the large offset vessel 310 is A3+A4. In oneapplication, the distance A3 is about 2400 m.

In another application, the front sources 304 and 306 may be located onone side of the traveling path 314 while the large offset front sourcesmay be located on the other side of the traveling path 314. In stillanother application, an inline distance A5 between the front sources andthe large offset front sources is in the order of kms. Similar to theembodiment illustrated in FIG. 5, the front and large offset frontsources may be separated along the traveling direction by predetermineddistances and these distances may be the same or different.

In one exemplary embodiment, an inline distance between the first frontsource 304 a and the central source 302 a is equal to an inline distancebetween the first front source 304 a and the second front source 306 aand this inline distance is equal to an inline distance between thefirst large offset front source 308 a and the second large offset frontsource 310 a. This configuration may be applied to any of theembodiments illustrated in FIGS. 5 and 6. It is also noted that a largeoffset between a source and a traveling distance is considered in theindustry to be equal to or larger than 2 km. Further, it is noted thatthe first and second front sources, the central source and the first andsecond large offset front sources may be actuated simultaneously orsequentially during the survey for both embodiments of FIGS. 5 and 6.

Having discussed the novel configurations for seismic data acquisition,a method for implementing the seismic data acquisition is now discussedin the following embodiments. According to an exemplary embodimentillustrated in FIG. 7, there is a method for seismic data acquisitionthat includes a step 700 of towing plural streamers 212 with a streamervessel 202; a step 702 of towing a central source 214 with the streamervessel 202; a step 704 of towing first and second front sources 204 a,206 a, located in front of the plural streamers 212 along a travelingdirection 216 of the streamer vessel 202, with corresponding fronttowing vessels; and a step 706 of towing first and second large offsetfront sources 208 a, 210 a, located in front of the first and secondfront sources 204 a, 206 a along the traveling direction 216, withcorresponding large offset front towing vessels. The first and secondfront sources, the central source and the first and second large offsetfront sources are actuating simultaneously or sequentially during thesurvey.

One or more of the methods discussed above may be implemented in acomputerized system as shown in FIG. 8. Such a computerized system 300may receive, via the input/output interface 302, information pertinentto positions of the sources and/or streamers, the arc angle, the turningradius, the run-in length, the run-out length, etc. In addition, thecomputerized system 300 may include a processor 304 for processing theabove-noted data and for calculating, for example, the size of a cell.The interface 302 and the processor 304 are connected to a bus 306.Further, the computerized system 300 may include a memory 306 to storethe above-noted data, a display 310, a connection 312 to the streamersand/or the sources, and other elements common for a computerized systemor server as would be recognized by those skilled in the art.

The above-disclosed exemplary embodiments provide a system and a methodfor improving azimuth distribution for seismic data acquisition. Itshould be understood that this description is not intended to limit theinvention. On the contrary, the exemplary embodiments are intended tocover alternatives, modifications and equivalents, which are included inthe spirit and scope of the invention as defined by the appended claims.Further, in the detailed description of the exemplary embodiments,numerous specific details are set forth in order to provide acomprehensive understanding of the claimed invention. However, oneskilled in the art would understand that various embodiments may bepracticed without such specific details.

For example, although the embodiments illustrated above in FIGS. 2, 5and 6 depict streamers arranged substantially parallel to one another,other streamer configurations may be employed in any of theseembodiments. For example, the streamers may be distributed in adovetail-like shape in any of these embodiments. In one application, thestreamers are fanned in a horizontal plane (substantially parallel tothe water surface) so that they make an angle with each other. Toachieve this arrangement, birds may be located on each streamer as shownin FIG. 3, for maintaining the streamers at the desired positions. Thebirds are devices capable of maintaining a vertical and/or horizontalposition in water. According to other embodiments, one or more of thestreamers may include a curved portion, or may be entirely curved, asopposed to the illustrated straight streamers in FIGS. 2, 5 and 6. Otherstreamer geometries may also be used in accordance with the positioningof seismic acquisition elements shown in any of these variousembodiments. Although the features and elements of the present exemplaryembodiments are described in the embodiments in particular combinations,each feature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein. Further, it is noted thatthe above embodiments may be implemented in software, hardware or acombination thereof.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A method for acquiring seismic data, the methodcomprising: towing, with a vessel, plural streamers and a first sourcearray along an inline direction; towing, with second and third vessels,second and third source arrays, respectively, the second and thirdvessels being located at different cross-line positions relative to thefirst vessel along a cross-line direction, wherein the cross-linedirection is substantially perpendicular to the inline direction and ina water plane; and towing, with fourth and fifth vessels, fourth andfifth source arrays, respectively, the first to fifth vessels beinglocated in front of a rear portion of the plural streamers along theinline direction, wherein an offset distance between the fourth andfifth vessels along the cross-line direction is larger than an offsetdistance between the second and third vessels, and wherein at least oneof the second and third vessels has a different inline position than atleast one of the fourth and fifth vessels.
 2. The method of claim 1,wherein the first source and one of the second and third sources areoffset by a predetermined first distance along the inline direction. 3.The method of claim 2, wherein the fourth and fifth sources are offsetfrom each other by a predetermined second distance along the inlinedirection.
 4. The method of claim 1, wherein the fourth and fifthsources are located in front of the second and third sources along theinline direction.
 5. The method of claim 1, wherein the plural streamersare fanned out so that two adjacent streamers make with each other anangle larger than 0.2 degree.
 6. The method of claim 1, wherein at leasta streamer of the plural streamers has a curved portion in a verticalplane.
 7. The method of claim 1, wherein the second and third sourcesare separated by a given inline offset.
 8. A method for acquiringseismic data, the method comprising: towing, with a first vessel, afirst seismic source and plural streamers along an inline direction;towing, with second and third vessels, second and third seismic sources,respectively, along the inline direction and having respective secondand third cross-line offsets relative to the first vessel along across-line direction, wherein the cross-line direction is substantiallyperpendicular to the inline direction and in a water plane; and towing,with fourth and fifth vessels, fourth and fifth seismic sources,respectively, along the inline direction in front of the first, secondand third vessels, and having respective fourth and fifth cross-lineoffsets relative to the first vessel, wherein the first to third vesselstravel in front of a rear portion of the plural streamers, and whereinat least one of the second and third vessels has a different inlineposition than at least one of the fourth and fifth vessels.
 9. Themethod of claim 8, wherein a sum of the fourth and fifth cross-lineoffsets is larger than a sum of the second and third cross-line offsets.10. The method of claim 8, wherein at least one of the second and thirdvessels travels in front of the first vessel along the inline direction.11. The method of claim 8, wherein the second and the fourth vesselstravel on one side of the first vessel relative to the inline directionand the third and the fifth vessels travel on the other side of thefirst vessel relative to the inline direction.
 12. The method of claim8, wherein the second and the third vessels are sandwiched between thefourth and fifth vessels along the cross-line direction.
 13. The methodof claim 8, wherein at least two sources of the first to fifth sourcesare staggered relative to the inline direction.
 14. The method of claim8, wherein the first vessel is configured to tow a first plurality ofstreamers.
 15. The method of claim 14, wherein one of the second andthird vessels is configured to tow a second plurality of streamers. 16.The method of claim 8, wherein the first to fifth sources include airguns.
 17. The method of claim 8, wherein the first to fifth sourcesinclude marine vibrators.
 18. A method for acquiring seismic data, themethod comprising: towing, with first to fifth seismic vessels,corresponding first to fifth seismic sources, wherein, in a system ofaxes including an inline direction and a cross-line direction, which aresubstantially perpendicular to each other, and which form a planesubstantially parallel to the water surface, each vessel has a differentcross-line position; setting a cross-line distance between the secondand third vessels to be less than a cross-line distance between thefourth and fifth vessels; positioning the first to fifth vessels totravel ahead of a rear portion of plural streamers towed along theinline direction; and selecting at least one of the second and thirdvessels to have a different inline position than at least one of thefourth and fifth vessels.
 19. The method of claim 18, wherein at leasttwo seismic sources of the first to fifth sources have different inlinepositions.
 20. The method of claim 18, wherein each seismic source has adifferent inline position.